Power-up circuit responsive to supply voltage transients

ABSTRACT

A power-up circuit in a computer system drives a memory device such as a dynamic random access memory (DRAM) to an initial condition after the computer system is turned on or reset. The power-up circuit also advantageously drives the memory device into the initial condition upon detecting a transient such as a negative glitch in a supply voltage being provided to the memory device. The power-up circuit includes a voltage level detector which causes a power-up signal to be provided to the memory device upon detecting that the supply voltage is less than a threshold voltage of the memory device which is necessary for the memory device to operate in an operational state. The power-up circuit also includes a delay circuit which causes the power-up signal to be provided to the memory device upon detecting that the supply voltage is beginning to rise from a quiescent voltage and at least until an amount of time determined by an RC time constant of the memory device for the memory device to enter the initial condition has passed. In response to receiving the power-up signal, the memory device enters the initial condition.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 08/494,718, filed Jun. 26, 1995, now U.S. Pat. No. 5,557,579.

TECHNICAL FIELD

This invention is directed in general to power-up circuits, and, more particularly, to power-up circuits which drive an electronic device to an initial condition upon detecting a transient in a supply voltage being provided to the electronic device.

BACKGROUND OF THE INVENTION

After a computer system is turned on or reset, many electronic devices in the computer system need to be driven to an initial condition prior to being used. An example of such an electronic device is a memory device. A typical memory device has memory cells which use a variety of logic circuits such as latches, gates and flip-flops to support their operation. These logic circuits need to be driven to an initial condition before the memory device is used to store data. A typical memory device also has a variety of capacitively loaded nodes which need to be driven to an initial condition such as a particular voltage.

Many electronic devices need a certain amount of time to enter an initial condition before they are ready to be used. One such electronic device is a memory device referred to as a dynamic random access memory (DRAM). A typical DRAM has a variety of capacitively loaded nodes which need to be driven to an initial condition at a particular voltage. For example, a typical DRAM has a plurality of DRAM memory cells, each of which includes a capacitor and an NMOS transistor. Each capacitor in each DRAM memory cell has one plate of its own coupled to a source terminal of its respective NMOS transistor and another plate shared in common among the capacitors in all of the DRAM memory cells. This common capacitor plate is referred to as the cell plate, and it often needs to be driven to an initial condition at a particular voltage. Since the cell plate is capacitively loaded, it is a node which needs a certain amount of time to enter the initial condition at the particular voltage. Since the typical DRAM has a variety of capacitively loaded nodes, including the cell plate, it has an overall RC time constant which controls the amount of time it needs to enter the initial condition. In a typical DRAM, this amount of time is in the range of 50 μS.

Since many electronic devices need to be driven to an initial condition prior to their use, circuit designers have conventionally designed these electronic devices to respond to a signal referred to as a power-up signal by entering the initial condition. Also, since these electronic devices often need a certain amount of time to enter the initial condition, circuit designers have conventionally used a resistor-capacitor (RC) time delay circuit to provide the power-up signal. In a typical RC time delay circuit, a voltage on a capacitor in an RC network in the circuit begins rising as a function of an RC time constant of the RC network when the circuit detects a supply voltage rising from a quiescent voltage such as 0.0 volts. This rising supply voltage is indicative of a computer system which has just been turned on or reset. At the same time that the voltage on the capacitor is rising, the RC time delay circuit provides the power-up signal to an electronic device to drive the electronic device to an initial condition. When the voltage on the capacitor rises to a certain level, the RC time delay circuit trips and stops providing the power-up signal to the electronic device. Circuit designers set the RC time constant of the RC network such that the RC time delay circuit provides the power-up signal to the electronic device for at least the amount of time the electronic device needs to enter the initial condition.

Although the conventional RC time delay circuit is generally adequate for driving an electronic device into an initial condition prior to its use, the present invention recognizes that the circuit is problematic for other reasons. The conventional RC time delay circuit cannot adequately detect a transient, such as a negative glitch, in a supply voltage being provided to an electronic device because the RC time constant of the RC network of the circuit makes the circuit too slow to react to the transient. This is a problem because, in many of the electronic devices the RC time delay circuit is used with, a transient in the supply voltage can cause the electronic device to enter an unknown state. In the case of one typical DRAM, for example, a transient in the supply voltage which drops the supply voltage below a threshold voltage of about 1.5 volts can cause a variety of latches, gates and flip-flops used to support the operation of memory cells in the DRAM to unlatch and lose latch data they are storing. This problem is exacerbated by a modern trend toward reducing power consumption in electronic devices by using lower supply voltages such as 3.3 volts instead of higher supply voltages such as 5.0 volts. With a lower supply voltage, a transient in the supply voltage is more likely to cause an electronic device to enter an unknown state.

Therefore, there is a need in the art for a circuit which drives an electronic device to an initial condition prior to its use and which drives an electronic device to an initial condition upon detecting a transient in a supply voltage being provided to the electronic device.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for driving an electronic device such as a memory device to an initial condition by providing a power-up signal to the electronic device. The electronic device has an RC time constant and is of the type to respond to the power-up signal by entering the initial condition after an amount of time determined by the RC time constant. The electronic device is also of the type to operate in an operational state when it receives a supply voltage greater than a threshold voltage and to be off when it receives a supply voltage equal to a quiescent voltage. A preferred embodiment of the present invention includes a power-up circuit having a voltage level detector providing a first reset signal in response to detecting that the supply voltage is less than the threshold voltage. The power-up circuit also has a delay circuit providing a second reset signal in response to detecting that the supply voltage is beginning to rise from the quiescent voltage and at least until the amount of time determined by the RC time constant of the electronic device for the electronic device to enter the initial condition has passed. The power-up circuit further has a logic circuit coupled to the voltage level detector and the delay circuit. The logic circuit provides the power-up signal to the electronic device in response to receiving at least one of the first and second reset signals.

The present invention thus advantageously drives an electronic device such as a memory device to an initial condition upon detecting a transient such as a negative glitch in a supply voltage being provided to the electronic device.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a block diagram of a preferred computer system according to the present invention.

FIG. 2 is a block diagram showing the memory device of the preferred computer system of FIG. 1 in more detail.

FIG. 3 is a block diagram showing the power-up circuit of the preferred computer system of FIGS. 1 and 2 in more detail.

FIG. 4 is a schematic and logic diagram of the preferred power-up circuit of FIG. 4.

FIG. 5 is a flow diagram showing a method for driving an electronic device into an initial condition according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, shown in FIG. 1, a computer system 10 includes an input device 12 such as a keyboard, an output device 14 such as a CRT monitor, and a storage device 16 such as a disk drive, all coupled to a conventional processor 18. The computer system 10 further includes a memory device, such as a dynamic random access memory (DRAM) device 20, coupled to the processor 18. Although the present invention will be described with respect to this preferred embodiment which includes a DRAM device, it will be understood by those having skill in the field of this invention that the present invention includes within its scope any electronic device, including any memory device.

The preferred DRAM device 20 of FIG. 1 is shown in more detail in FIG. 2. The DRAM device 20 includes a power-up circuit 22 coupled to a DRAM circuit 24 to provide a power-up signal 26 to the DRAM circuit 24. Both the power-up circuit 22 and the DRAM circuit 24 receive a supply voltage V_(cc). The DRAM circuit 24 is a typical DRAM as discussed above which is constructed to respond to the power-up signal 26 by entering an initial condition after an amount of time determined by an RC time constant of the DRAM circuit 24. The DRAM circuit 24 is also constructed such that it requires the supply voltage V_(cc) to be greater than a threshold voltage in order for it to operate in an operational state.

The preferred power-up circuit 22 of FIG. 2 is shown in more detail in FIG. 3. The power-up circuit 22 includes a voltage level detector 28 which detects that the supply voltage V_(cc) is less than the threshold voltage of the DRAM circuit 24 and provides a first reset signal 30 to a logic circuit 32 in response. This condition is referred to as a negative glitch in the supply voltage V_(cc), and it would normally occur only after the DRAM circuit 24 has begun operating in its operational state and the supply voltage V_(cc) has risen to an operational value such as 3.3 volts. The power-up circuit 22 also includes a delay circuit 34 which provides a second reset signal 36 to the logic circuit 32 in response to detecting that the supply voltage V_(cc) is beginning to rise from a quiescent voltage such as 0.0 volts. The delay circuit 34 has a time constant that causes the second reset signal 36 to have a duration at least as long as the amount of time required for the DRAM circuit 24 to enter the initial condition. This situation would normally occur when the preferred computer system is turned on or reset. In response to receiving at least one of the first and second reset signals 30 and 36, the logic circuit 32 provides the power-up signal 26 to the DRAM circuit 24. The preferred power-up circuit 22 thus advantageously drives the DRAM circuit 24 to the initial condition prior to its use and upon detecting a negative glitch in the supply voltage V_(cc).

In an alternative version of the preferred power-up circuit 22, the voltage level detector 28 provides the first reset signal 30 not only to the logic circuit 32 but also to the delay circuit 34 in response to detecting that the supply voltage V_(cc) is less than the threshold voltage of the DRAM circuit 24. In response to receiving the first reset signal 30, the delay circuit 34 provides the second reset signal 36 to the logic circuit 32 for a duration at least long enough for the DRAM circuit 24 to enter the initial condition. In turn the logic circuit 32 provides the power-up signal 26 to the DRAM circuit 24 in response to receiving the second reset signal 36. In this manner the described alternative version of the preferred power-up circuit 22 ensures that the power-up signal 26 drives the DRAM circuit 24 to the initial condition after a negative glitch in the supply voltage V_(cc) is detected.

A schematic and logic diagram of the preferred power-up circuit 22 of FIGS. 2 and 3 is shown in FIG. 4. In operation, the supply voltage V_(cc) is initially at a quiescent voltage such as 0.0 volts. When the preferred computer system is turned on or reset, the supply voltage V_(cc) begins to rise from the quiescent voltage. At the same time, a divider voltage V_(d) equal to a fraction of the supply voltage V_(cc) also begins to rise at a tap terminal 38 of a voltage divider 40 of the voltage level detector 28. The voltage divider 40 includes three PMOS transistors P₁, P₂ and P₃ configured as resistive devices between the tap terminal 38 and the supply voltage V_(cc) and another PMOS transistor P₄ configured as a resistive device between the tap terminal 38 and ground.

When the supply voltage V_(cc) rises above a threshold voltage of an NMOS transistor N₁ of the voltage level detector 28, the transistor N₁ turns on and pulls its drain close to ground, thereby providing the first reset signal 30. At the same time, a PMOS transistor P₅ of the voltage level detector 28 is off because the drop from the supply voltage V_(cc) to the divider voltage V_(d) is less than a threshold voltage of the transistor P₅. When the first reset signal 30 is provided to a first input terminal 42 of a NAND gate 43 of the logic circuit 32, it causes the NAND gate 43 to provide a high voltage at a NAND gate output terminal 44. Because the NAND gate output terminal 44 is a logic high signal, an NMOS transistor N₂ of the voltage level detector 28 turns on and couples the first input terminal 42 of the NAND gate 43 to ground through a series of NMOS transistors N_(R) of the voltage level detector 28. The transistor N₂ thereby provides hysteresis to assist the transistor N₁ in pulling the first input terminal 42 of the NAND gate 43 to ground and hence in providing the first reset signal 30. The NAND gate output terminal 44 provides its logic high signal to an inverter I of the logic circuit 32 which in turn provides a logic low power-up signal at an output terminal 46 as the power-up signal 26. Because the output terminal 46 of the inverter I has a logic low level, it turns a PMOS transistor P₆ of the voltage level detector 28 on and couples the tap terminal 38 to the supply voltage V_(cc). The transistor P₆ thereby provides additional hysteresis in pulling the tap terminal 38 to a logic high level.

As the supply voltage V_(cc) continues to rise, the divider voltage V_(d) also continues to rise. When the drop from the supply voltage V_(cc) to the divider voltage V_(d) is greater than the threshold voltage of the transistor P₅, the transistor P₅ turns on. At this time, the drain of the transistor P₅ rises almost to the supply voltage V_(cc) and terminates the first reset signal 30. This occurs despite the transistor N₁ being on because the transistor P₅ is constructed to have a much smaller resistance than the transistor N₁. Because the drain of the transistor P₅ rises almost to the supply voltage V_(cc), the input terminal 42 of the NAND gate 43 also rises almost to the supply voltage V_(cc).

As mentioned above, the delay circuit 34 also generates a reset signal when the supply voltage V_(cc) increases from a quiescent voltage. With reference to the upper portion of FIG. 4, when the supply voltage V_(cc) first begins to rise from the quiescent voltage, a switching voltage V_(s) at an output terminal 48 of an RC circuit of the delay circuit 34 also begins to rise. The RC circuit comprises a PMOS transistor P₇ configured as a resistive device and a capacitor C₁. Because the output terminal 48 of the RC circuit is capacitively loaded by the capacitor C₁, the switching voltage V_(s) will not rise as quickly as the supply voltage V_(cc). As a result, as the supply voltage V_(cc) rises, the drop from the supply voltage V_(cc) to the switching voltage V_(s) exceeds a threshold voltage of a PMOS transistor P₈ of the delay circuit 34. When this occurs, the transistor P₈ turns on and pulls an input terminal 50 of a first buffer device of the delay circuit 34 comprising PMOS and NMOS transistors P₉ and N₃, respectively, up to the supply voltage V_(cc). A capacitor C_(b) of the delay circuit 34 assists the transistor P₈ in pulling the input terminal 50 of the first buffer device up to the supply voltage V_(cc). Because the input terminal 50 of the first buffer device is at the supply voltage V_(cc), the transistor N₃ turns on and pulls an input terminal 52 of a second buffer device of the delay circuit 34 comprising PMOS and NMOS transistors P₁₀ and N₄ down to ground. Because the input terminal 52 of the second buffer device is at ground, the transistor P₁₀ turns on and pulls an input terminal 54 of a third buffer device of the delay circuit 34 comprising PMOS and NMOS transistors P₁₁ and N₅ up to the supply voltage V_(cc). Because the input terminal 54 of the third buffer device is at the supply voltage V_(cc), the transistor N₅ turns on and pulls an output terminal 56 of the third buffer device down to ground, thereby providing the second reset signal 36 to a second input terminal 58 of the NAND gate.

As the supply voltage V_(cc) continues to rise, the switching voltage V_(s) rises sufficiently to turn on a diode-configured NMOS transistor N₆ and an NMOS transistor N₇ of the delay circuit 34. When the drop from the supply voltage V_(cc) to the switching voltage V_(s) falls below the threshold voltage of the transistor P₈, an NMOS transistor N₈ of the delay circuit 34 turns on and the transistor P₈ turns off. When this occurs, the input terminal 50 of the first buffer device is pulled to ground by the transistor N₈, the input terminal 52 of the second buffer device is pulled up to the supply voltage V_(cc) by the transistor P₉, the input terminal 54 of the third buffer device is pulled to ground by the transistor N₄, and the output terminal 56 of the third buffer device is pulled up to the supply voltage V_(cc) by the transistor P₁₁, thereby terminating the second reset signal 36. Because the input terminal 52 of the second buffer device is pulled up to the supply voltage V_(cc), an NMOS transistor N₉ of the delay circuit 34 turns on and assists the transistor N₈ in pulling the input terminal 50 of the first buffer device down to ground, thereby providing hysteresis. Also, because the time at which the switching voltage V_(s) turns the transistor P₈ off and the transistor N8 on is a function of the resistance of the transistor P₇ and the capacitance of the capacitor C₁, these values are set such that the second reset signal 36 is terminated after an amount of time at least equal to the amount of time required for the DRAM circuit (FIG. 3) to enter a an initial condition.

When the first and second reset signals 30 and 36 are terminated and the first and second input terminals 42 and 58 of the NAND gate are at the supply voltage V_(cc), the NAND gate 43 provides a low voltage at its output terminal 44 to the inverter I, which in turn provides a high voltage at its output terminal 46, thereby terminating the power-up signal 26. At the same time, the low voltage at the NAND gate output terminal 44 and the high voltage at the inverter I output terminal 46 turn off the transistors N₂ and P₆, respectively, and thereby provide hysteresis.

During subsequent operation, if a transient such as a negative glitch occurs on the supply voltage V_(cc) which is sufficient to make the drop from the supply voltage V_(cc) to the divider voltage V_(d) less than the threshold voltage of the transistor P₅, then the transistor P₅ turns off and the transistor N₁ again pulls the first input terminal 42 of the NAND gate down to ground and thereby provides the first reset signal 30. Operation of the preferred power-up circuit 22 when the first reset signal 30 is provided is described above. The preferred power-up circuit 22 thus advantageously provides the power-up signal 26 upon detecting a negative glitch in the supply voltage V_(cc). Also, when the preferred computer system is subsequently turned off and the supply voltage V_(cc) drops to the quiescent voltage, a diode-connected NMOS transistor N₁₀ of the delay circuit 34 turns on, causing the capacitor C₁ to discharge back to the supply voltage V_(cc) through the transistors N₆ and N₁₀.

In another embodiment, the present invention provides a method in a computer system for driving an electronic device into an initial condition as shown in FIG. 5. In an initial step 60, the method includes detecting that a supply voltage is beginning to rise from a quiescent voltage. In another step 62, the method includes initiating providing a power-up signal to the electronic device in response to detecting that the supply voltage is beginning to rise from the quiescent voltage. In a further step 64, the method includes detecting that the supply voltage has been greater than the quiescent voltage for a predetermined amount of time and that the supply voltage is greater than a threshold voltage. In a still further step 66, the method includes terminating providing the power-up signal to the electronic device in response to detecting that the supply voltage has been greater than the quiescent voltage for the predetermined amount of time and in response to detecting that the supply voltage is greater than the threshold voltage. In this manner the electronic device begins to operate in an operational state. In another step 68, the method includes detecting that the supply voltage is less than the threshold voltage after a period of time in which the electronic device operates in the operational state. As a result, the electronic device discontinues operating in the operational state. In still another step 70, the method includes providing the power-up signal to the electronic device in response to detecting that the supply voltage is less than the threshold voltage.

Although the present invention has been described with reference to a preferred embodiment, the invention is not limited to this preferred embodiment. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described. 

We claim:
 1. A power-up circuit for generating a power-up signal for an electronic device adapted to be powered by a supply voltage, the power-up circuit comprising:a voltage level detector coupled to the supply voltage, the voltage level detector providing a first signal in response to detecting that the absolute value of the supply voltage is less than a threshold voltage; a delay circuit coupled to the supply voltage, the delay circuit providing a second signal for the duration of a delay period after the absolute value of the supply voltage has risen above a quiescent voltage; and a logic circuit coupled to the voltage level detector and the delay circuit, the logic circuit providing the power-up signal to the electronic device until the logic circuit receives neither of the first and second signals.
 2. The power-up circuit of claim 1 wherein the delay circuit is coupled to the voltage level detector and receives the first signal, wherein the delay circuit provides the second signal for a predetermined period in response to receiving the first signal.
 3. The power-up circuit of claim 1 wherein the voltage level detector comprises:a voltage divider receiving the supply voltage and providing a divider voltage which is a fraction of the supply voltage; and a first switching device having a control terminal coupled to the voltage divider and receiving the divider voltage, the first switching device being configured to provide the first signal when the received divider voltage is such that the supply voltage is less than the threshold voltage.
 4. The power-up circuit of claim 1 wherein the delay circuit comprises:as buffer having an input terminal and an output terminal, the input terminal of the buffer receiving a switching voltage, the output terminal of the buffer providing the second signal; and an RC circuit having an output terminal coupled to the input terminal of the buffer and providing the switching voltage to the buffer, the RC circuit including a capacitive device and a switching device configured as a resistive device coupled to one another at the output terminal of the RC circuit in series between the supply voltage and ground.
 5. The power-up circuit of claim 1 wherein the logic circuit comprises an AND gate having first and second input terminals coupled to the voltage level detector and the delay circuit, respectively, and further having an output terminal providing the power-up signal to the electronic device.
 6. The power-up circuit of claim 1 wherein the electronic device is a memory device.
 7. A power-up circuit for generating a power-up signal for an electronic device adapted to be powered by a supply voltage, the power-up circuit comprising:a voltage level detector coupled to the supply voltage, the voltage level detector providing a first signal in response to detecting that the absolute value of the supply voltage is less than a threshold voltage; a low pass filter circuit coupled to the supply voltage, the low pass filter circuit low pass filtering the supply voltage to generate a low pass filtered supply voltage signal, the low pass filter circuit providing a second signal for as long as the absolute value of the low pass filtered supply voltage signal has a magnitude that is smaller than a predetermined value; and a logic circuit coupled to the voltage level detector and the delay circuit, the logic circuit providing the power-up signal to the electronic device in response to receiving either of the first and second signals.
 8. The power-up circuit of claim 7 wherein the voltage level detector comprises:a voltage divider receiving the supply voltage and providing a divider voltage which is a fraction of the supply voltage; and a first switching device having a control terminal coupled to the voltage divider and receiving the divider voltage, the first switching device being configured to provide the first signal when the received divider voltage is such that the supply voltage is less than the threshold voltage.
 9. The power-up circuit of claim 7 wherein the a low pass filter circuit comprises:an RC circuit having an output terminal providing the low pass filtered supply voltage signal, the RC circuit including a capacitive device and a semiconductor device configured as a resistive device coupled to one another at the output terminal of the RC circuit in series between the supply voltage and ground; and a buffer having an input terminal and an output terminal, the input terminal of the buffer receiving the switching voltage from the RC circuit, the output terminal of the buffer providing the second signal.
 10. The power-up circuit of claim 7 wherein the logic circuit comprises an AND gate having first and second input terminals coupled to the voltage level detector and the delay circuit, respectively, and further having an output terminal providing the power-up signal to the electronic device.
 11. The power-up circuit of claim 7 wherein the electronic device is a memory device.
 12. A power-up circuit for generating a power-up signal for an electronic device adapted to be powered by a supply voltage, the power-up circuit comprising:a voltage level detector coupled to the supply voltage, the voltage level detector providing a first signal in response to detecting that the absolute value of the supply voltage has a magnitude that is greater than a threshold voltage; a delay circuit coupled to the supply voltage, the delay circuit providing a second signal responsive to the absolute value of the supply voltage being larger than a quiescent voltage for longer than a predetermined duration; and a logic circuit coupled to the voltage level detector and the delay circuit, the logic circuit providing the power-up signal to the electronic device in response to receiving both of the first and second signals.
 13. The power-up circuit of claim 12 wherein the delay circuit is coupled to the voltage level detector and receives the first signal, wherein the delay circuit provides the second signal for a predetermined period in response to receiving the first signal.
 14. The power-up circuit of claim 12 wherein the voltage level detector comprises:a voltage divider receiving the supply voltage and providing a divider voltage which is a fraction of the supply voltage; and a first switching device having a control terminal coupled to the voltage divider and receiving the divider voltage, the first switching device being configured to provide the first signal when the received divider voltage is such that the supply voltage is less than the threshold voltage.
 15. The power-up circuit of claim 12 wherein the delay circuit comprises:a buffer having an input terminal and an output terminal, the input terminal of the buffer receiving a switching voltage, the output terminal of the buffer providing the second signal; and an RC circuit having an output terminal coupled to the input terminal of the buffer and providing the switching voltage to the buffer, the RC circuit including a capacitive device and a switching device configured as a resistive device coupled to one another at the output terminal of the RC circuit in series between the supply voltage and ground.
 16. The power-up circuit of claim 12 wherein the logic circuit comprises an AND gate having first and second input terminals coupled to the voltage level detector and the delay circuit, respectively, and further having an output terminal providing the power-up signal to the electronic device.
 17. The power-up circuit of claim 12 wherein the electronic device is a memory device. 